Analysing activation products

DISMANTLING AND DECOMMISSIONING OF A nuclear power plant produces several hundred thousand tonnes of debris. Only a few percent of this waste has to be disposed of as radioactive waste. Researchers from Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) and the University of Cologne are working to establish a new way of measuring the waste arising from the decommissioning, which allows for a more precise and reliable determination of the type and quantity of radioactive components.

During plant operations, certain components and structures are contaminated with radionuclides. In addition to surface contamination, activation also occurs, by which originally non-radioactive substances are converted into radionuclides. This process leads, for example, to the formation of calcium-41 in the biological shield, a concrete structure 1-2m thick that surrounds the reactor pressure vessel. Part of the concrete has thus to be disposed of as radioactive waste after decommissioning.

It is important to know as precisely as possible which radionuclides are present in which quantities in plant components – for example, for radiation protection planning or for the selection of methods for the decontamination of components. This radioactive inventory is also key in determining which materials arising from dismantling can be reused or disposed of as conventional waste and which are to be classified as radioactive waste. Measurements play an essential role in determining the radioactive inventory.

Assessing surface contamination

Measurements of surface contamination usually do not present any problems. However, measuring radionuclides within activated structures is more difficult. Only radionuclides emitting gamma radiation can be measured on site. To determine the remaining nuclides, so-called reference nuclides and nuclide vectors are used. This is based on the knowledge that gamma emitters and other radionuclides that are more difficult to measure are present in a certain proportion in certain cases – for example in the wall of the biological shield. The quantity of the remaining radionuclides can thus be determined from the quantity of the easily measurable reference nuclide. To determine the nuclide vectors, detailed investigations of samples of activated materials are used, as well as extensive calculations based on the decay series of relevant radionuclides and their half-lives. This ensures that those radionuclides that are difficult to measure are also considered by the nuclide vectors, leading to conservative results on radiological relevance.

Under certain circumstances there are limits to the use of reference nuclides and nuclide vectors. If the reference nuclide has a short half-life and decays faster than the other nuclides of the vector, after only a few years the remaining quantity of reference nuclide may be too small to be measured. A similar problem arises if only one element, which from the start is only present in very small traces, is considered as a reference nuclide.

Researchers at GRS have set themselves the goal of investigating the radioactive inventory of small amounts of activated concrete using one of the most accurate methods currently available. In a project funded by Germany’s Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, and with scientists of the Department of Chemistry and the Institute for Nuclear Physics of the University of Cologne, they are using accelerator mass spectrometry (AMS).

In this process a sample is first converted into an ion beam. The ions with the same mass as the required radionuclide are separated from the beam in a mass spectrometer. However, the remaining ions may be single atoms or ionised molecules with the same mass. This is where the accelerator comes into play: it ensures that all molecular compounds are destroyed, so only one beam of individual ionised atoms remains. This beam is then filtered again in a second mass spectrometer using the mass, so that at the end only the required nuclides are counted in a detector.

This method allows extremely precise measurements to be made. AMS could be used to detect a single radionuclide in up to 10 quadrillion other non-radioactive nuclides. The idea of using this measurement method for the investigation of waste arising from the decommissioning of nuclear power plants was inspired by the history of the Ötzi ice mummy.

Physicist Matthias Dewald, who manages the project at GRS, explained, “An accelerator mass spectrometer was used to measure the amount of carbon-14 in Ötzi’s remains. In the past, methods such as liquid scintillation, which was also used in nuclear technology, were used for such measurements. In the meantime, AMS has become the method of choice for age determination or trace analysis in climate research – the procedure is simply much more precise and less complex than the others. That’s why it made sense for us to apply this method to our investigations as well”.

Research reactor as time lapse

In one of the two AMS plants at the University of Cologne, researchers are evaluating concrete samples that had previously been irradiated in the Triga research reactor in Mainz. “We can achieve an activation of the samples within seconds to a few minutes as it is only achieved in a nuclear power plant after many years of operation. The Triga is something of a time-lapse for us,” says Dewald. To generate different concentrations of the wanted radionuclides in the samples, the duration of irradiation was varied. As a target, the scientists set themselves a concentration of Ca-41 ranging between one to 10 billion and one to one trillion. This corresponds approximately to the concentration found in the biological shield after the decommissioning of a nuclear power plant.

In addition to the AMS measurements, some of the samples are also evaluated using gamma spectroscopy. This method takes advantage of the fact that every radionuclide emits gamma rays in a very specific energy range. Scientists can use this “energetic fingerprint” to determine which gamma emitters are present in the samples and in what proportions.

Among the uses, in combination with the results of the AMS measurements, it will be possible to expand the knowledge base for the development of nuclide vectors and hence to clarify whether other, very long-lived radionuclides such as Ca-41 can also be considered as reference nuclides. 

First new findings

Dewald says of the first results of the analysis: “We have already been able to determine first differences between the radionuclide compositions and nuclide vectors measured by us, which are known to us from the literature. How significant these differences are, however, is not yet possible to determine – work still lies ahead for us.”

However, Dewald has no doubts that the “project contributes to improving the methodological basis and to extending the validation of nuclide vectors”. In the next phase of work, samples from the biological shields of two decommissioned reactors – Mehrzweckforschungsreaktor Karlsruhe (a 57MW pilot PWR cooled and moderated by heavy water) and the Kompakte Natriumgekühlte Kernreaktoranlage Karlsruhe (a sodium-cooled test reactor) – will be examined to confirm the results of the previous investigations. The project is to be completed by the end of 2019. A detailed report is to be published in early 2020.

In the longer term, the team is aiming for a change of material, investigating activated graphite used in reactors. This is not only used RBMKs, but also in Germany in the high-temperature reactor THTR-300 in Hamm, in several research reactors and also in the UK’s fleets of Magnox and Advanced Gas Cooled Reactors.